1. Field of the Invention
The present invention relates to improvements in a hydraulic control system for a continuously variable transmission of a belt-and-pulley type for an automotive vehicle.
2. Discussion of the Related Art
A belt-and-pulley type of continuously variable transmission for an automotive vehicle is known. This transmission includes a first shaft, a second shaft, a first variable-diameter pulley provided on the first shaft, a second variable-diameter pulley provided on the second shaft, a transmission belt connecting the first and second pulleys to transmit power from the first shaft to the second shaft, a first hydraulic cylinder for changing an effective diameter of the first pulley, and a second hydraulic cylinder for changing an effective diameter of the second pulley. An example of a hydraulic control system for controlling the speed ratio and belt tension of this type of continuously variable transmission is disclosed in Publication No. 58-29424 (published in 1983 for opposition purpose) of Japanese Patent Application, wherein a shift-control valve (four-way valve) is provided for feeding a pressurized working fluid from a hydraulic pressure source to one of the two hydraulic cylinders and allowing the fluid to be discharged from the other cylinder. The control system further uses a solenoid-operated relief valve (pressure regulating valve) for regulating the pressure of the fluid which is delivered from the shift-control valve.
In the hydraulic control system indicated above, a comparatively high pressure generated by the pressure source is applied to the hydraulic cylinder for the pulley provided on the drive shaft, i.e., to one of the two cylinders which is subject to a relatively high pressure during power transmission in the direction from the drive shaft to the driven shaft of the transmission. The other cylinder is supplied with the pressure which is regulated by the pressure regulating valve. According to this arrangement, the belt tension and the speed ratio of the transmission can be suitably controlled, even when the direction of power transmission is reversed.
3. Problems Solved by the Invention
In the continuously variable transmissions as discussed above, the working fluid usually leaks to some extent through hydraulically operated members such as the hydraulic cylinders. The rate of leak flows of the fluid from the cylinders may increase to an abnormal level when the fluid temperature is elevated, or if the sealing members for the cylinders are deteriorated, or if foreign matters enter the hydraulic system. In this event, the pressure in the low-pressure cylinder provided on the driven shaft of the transmission may fall below a lower limit necessary to maintain a minimum required tension of the transmission belt. In this case, therefore, the belt may slip on the pulleys, leading to reduced durability of the continuously variable transmission.
Further, the conventional transmissions require complicated control formulas for controlling the pressure regulating valve to regulate the pressure of the fluid which flows from the low-pressure cylinder to the shift-control valve. Moreover, the pressure regulating valve is uncapable of exact or accurate control of the pressure. Stated more specifically referring to FIG. 4, there exists a pressure difference (.DELTA.P2 in the figure) between the pressure (second line pressure Pl2 in the figure) established by the pressure regulating valve by controlling the pressure of the fluid which is discharged from the low-pressure cylinder and returned to the shift-control valve, and the pressure (Pin or Pout in the figure) in the low-pressure cylinder, while the shift-control valve is placed near its neutral position. Therefore, the control value or signal for controlling the pressure regulating valve to regulate the pressure of the fluid discharged from the low-pressure cylinder must be compensated for the above pressure difference, which must be calculated by using a suitable equation. Since the equation for accurate calculation of the pressure difference is very much complicated, the difference is usually obtained by approximation by using a relatively simple equation. This results in accordingly inaccurate compensation for the actually existing pressure difference. Even if the pressure difference can be obtained accurately, there exists a difference in operating characteristics of the individual pressure regulating valves. For these reasons, the pressure of the fluid discharged from the low-pressure cylinder cannot be optimally controlled by the pressure regulating valve.